This improves a regression added in b9b55cec4403f69d999ff6c3b90472805dfbed71
that reduced performance for large number of options when using a
jQuery collection object to append multiple options instead of
using a bare array containing the jQuery elements.
As `$.fn.add` is only required for jQuery 1.7.x, we can use a
utility function that only falls back to it for that specific
version of jQuery, and uses `$.fn.append` with an array for all
other versions.
This closes https://github.com/select2/select2/issues/3230.
4fc874a3ae